Your hour meter there either has a metal housing case cover ground, or else the ground wire was ripped out forcefully, and that missing grd wire was physically ripped out of the sealed hour meter body case cover itself - which may or may not be made out of metal.
The 1 wire you have showing there is no doubt RED in color - hopefully? If it's BLACK or GREEN then the HOT (Magneto) wire was ripped out of the meter case for whatever reason.
Every electronic meter today has at least a 2-wire Battery Hot & Ground wire scheme whether or not the Batt Hot is key switched or not . Has to - otherwise the meter won't work.
Some gauge meters might even have a 3rd or 4th wire scheme as well - beings a SENSE and/or a keyed IGN wire scheme - just depends on the type meter and it's lighting/switched scheme.
Also depends on the meter design if it's using an inductive type SENSE pickup signal as well. Other then the BLUE wire (same as like on Marine apps) going to the Inst panel LIGHT switch, and the BLACK wire going directly to Grd, the only other wire(s) left are the HOT or RED WIRE which almost always goes to the IGN side of the Key Switch, and most likely in your tractor case (if it's an older design) that goes to the Magneto. The Grd wire always goes to a good Batt return Grd point anywhere on the tractor.
When you buy used stuff - esp junk off Ebay - you are taking a "Pig in a Poke" chance.
Does your meter have a replaceable 12-volt light bulb socket in the back? If it does then it has to have an external Grd somewhere. If it's a Super Bright White LED type backlight then it may be taking it's Grd internally from the meter itself. In either case there still needs to be a good Batt Grd return for the hour meter.
If your meter there truly only has the 1 wire (RED or GREEN/BLACK) coming out of the sealed meter case (usually comes thru a rubber type wiring seal grommet), and the meter case is non-metal (in other words a plastic case) - then you only have one option left, and that is to physically pry open the meter case and re-establish (resolder) either a good HOT wire or Grd point inside. That is - If you are capable of doing that without destroying the case? I've done it before for people, but again I am me, and you are you, and I know I can do it, but I don't know if you can.
If the meter has a metal face ring that might be a good Grd point. Only way to check it is by hooking up the (hopefully RED lead) to Batt + and touching the metal face ring to Batt Grd to see if it even fires up or its motor runs. If it's an inductive type meter scheme (ie Magneto type), and there only is a RED wire present with the metal face ring being Grd - then you might have to hook it up to the running tractor's Magneto point to see if it truly works. It may very well be that you wasted money on this possible "AS IS" Ebay buy there? Did you get it off Ebay? If you did, and there were only shaddy looking pix of the meter at best - then I would have passed on that USED deal. Sometimes a pix says it all.
Hope that helps - let me know?
Frank
